I started with Good Sam. Dropped them like a hot potato because of their then-over-the-top sales practices (since toned down, I'm told). Then went to Allstate Motor Club (same provider as GS and used to be cheaper). Eventually got uncomfortable over an ambiguity about whether or not they were promising to pay for a tow to an actual Mercedes dealer. I could never get a straight answer from them, so I decided to pay a few more bucks and move to Coach-Net, which as obgraham says, has an impeccable reputation.
Never actually used any of them.
BTW: many of these plans covers ALL you vehicles, which permits you to drop AAA (IMO a very good thing for more reasons than one).